The field of the invention is r.f. sputtering and said invention is especially concerned with the type of r.f. sputtering in which a layer or coating of material is sputtered onto a surface of an elongate web of flexible substrate moving linearly and continuously through a sputtering chamber.
With respect to the background of the invention, most sputtering apparatus is of the batch variety, that is, of a type in which a sputtering vessel is closed with a substrate member sealed inside; the sputtering conditions are established; the substrate member is moved through the sputtering plasma and coated; the sputtering conditions are discontinued; and the vessel is opened, the substrate member removed and a new one is installed therein. Then the process is repeated. Even apparatus which is constructed to enable sputtering of a relatively long substrate member, say of the length of several hundred meters, played off a supply reel and wound onto a take-up reel all on the interior of a vessel is a batch device. Time is lost in opening and closing the vessel, reestablishing the sputtering conditions, retuning the electronics, etc. Notwithstanding great effort to duplicate conditions from batch to batch, uniformly is difficult to achieve. Furthermore, variations in the ambient atmosphere from batch-to-batch affect the coatings because of the need for exposure during installation and removal of the substrate member.
Advantages are achieved in moving a substrate member that is in the form of an elongate web through apparatus which can continue to sputter-coat the substrate member under uniform conditions so long as the substrate is passing through. The apparatus is an elongate vessel which forms a sealed chamber, and the substrate member enters and exits by way of air locks which are maintained at low pressure by means of suitable pumps. The movement is called rectilinear because the general effect is to transport the substrate in a straight line, but there may be and usually are drive means in the apparatus which comprise rollers at least partially around which the substrate may move.
The targets and anodes are arranged parallel to one another so that sputtering gaps are established in the spaces between. The substrate is passed through these gaps in contact with the anodes and moves in a plane parallel to those of the anode and target faces.
One of the problems which arise in the construction of apparatus of this type is the difficulty of access to the electrodes for replacement and cleaning. Another problem is the difficulty of threading the substrate member through the apparatus. Another problem is that the apparatus is expensive if it is dedicated to only one type of substrate and one type of target material.
It is often desired to sputter an elongate substrate member with one or more stripes of sputtered material. This presents the problems of how to do it economically and with simple apparatus; how to be able to change the size and location of the stripes; how to use the same apparatus for non-stripe sputtering.
Another set of problems arises when one wishes to sputter substrate material that is heat-sensitive. Paper and various plastics will scorch and even be destroyed during sputtering because the electrons that are produced in the plasma and which do not contribute to the coating are driven against the substrate and heat it inordinately. Documents of value are capable of being made of paper or other sheeting that has been tagged with bands or stripes of non-visible material sputtered thereon. Genuineness and denomination can be identified from such tagging, but the tagging cannot be done if the sheeting material is burnt by the method of application.
The invention is directed to the solution of these and many other problems.